1. Field of the Invention
The present invention relates to an X-ray mask employed for X-ray lithography and a method of fabricating the same, and more concretely, it relates to an X-ray mask which makes it possible to reduce distortion in patterning by changing the crystal structure or stress of an etching mask or an antireflection film (etching stopper) and a method of fabricating the same.
2. Description of the Background Art
In semiconductor memory devices having relatively low degrees of integration, patterns have generally been transferred through lithography with ultraviolet rays. However, semiconductor memory devices are now implemented with higher degrees of integration, and a semiconductor memory device in the gigabit range such as a 1-gigabit DRAM (dynamic random access memory), for example, requires pattern transfer of higher resolution since patterns of wires and the like are extremely refined in accordance with device rules.
Lithography with X-rays is expected as the technique for transferring such fine patterns. According to this X-ray lithography, patterns can be transferred in higher resolution than the lithography with ultraviolet rays since X-rays serving as exposure light have shorter wavelengths (.lambda.=5 to 20 nm in case of soft X-rays) as compared with ultraviolet rays.
A structure of an X-ray mask employed for such X-ray lithography and a method of fabricating the same are described in SPIE (The Society of Photo-Optical Instrumentation Engineers), 1994, vol. 2194, pp. 221 to 230, for example. The structure of the X-ray mask and the method of fabricating the same shown in this literature are now described as the prior art.
FIG. 8 is a sectional view schematically showing the structure of the conventional X-ray mask described in the aforementioned literature. Referring to FIG. 8, the X-ray mask has a silicon substrate 1, a membrane 2, an etching stopper serving also as an antireflection film (hereinafter simply referred to as an antireflection film) 103a, an antireflection film 103b, an X-ray absorber 104, and a support ring 5.
The membrane 2, which is a substrate transmitting X-rays, is formed on the silicon substrate 1. The antireflection films 103a and 103b are formed on the front and rear surfaces of the membrane 2 respectively. The X-ray absorber 104, consisting of a material intercepting transmission of X-rays, is formed on the antireflection film 103a in a prescribed shape. The support ring 5 is stuck to the rear surface of the silicon substrate 1 with an adhesive (not shown), and supports the silicon substrate 1.
A method of fabricating the aforementioned conventional X-ray mask is now described.
FIGS. 9 to 11 are schematic sectional views showing the method of fabricating the conventional X-ray mask in order of the steps. Referring to FIG. 9, the membrane 2 is first formed on the silicon substrate 1, and thereafter a part of the silicon substrate 1 is removed (back-etched), to expose the rear surface of the membrane 2. Thereafter the antireflection films 103a and 103b are formed on the front and rear surfaces of the membrane 2 respectively. Thereafter the support ring 5 is stuck to the silicon substrate 1.
Referring to FIG. 10, the X-ray absorber 104 is formed on the antireflection film 103a by sputtering, for example. The current mean membrane stress of the X-ray absorber 104 is measured, a temperature for zeroing the mean stress is decided, and the X-ray absorber 104 is uniformly annealed in an oven at 250.degree. C., for example, so that its mean membrane stress is adjusted to zero. An etching mask 106 is formed on the X-ray absorber 104 by sputtering, for example. A resist material 7 is applied onto this etching mask 106, and thereafter baked at 180.degree. C., for example.
Referring to FIG. 11, a pattern is drawn on the resist material 7 with an electron beam drawer (EB), and thereafter developed to form a resist pattern 7. The etching mask 106 is etched through the resist pattern 7. Thereafter the X-ray absorber 104 is etched through the etching mask 106. Finally, the etching mask 106 is removed, to complete the X-ray mask shown in FIG. 8.
The steps of back-etching the silicon substrate 1 and sticking the silicon substrate 1 to the support ring 5 are not necessarily performed in this order.
In relation to a semiconductor memory device in the gigabit range requiring fine working, the resist pattern 7 cannot be increased in thickness. If the resist pattern 7 is directly formed on the X-ray absorber 104 for patterning the X-ray absorber 104, the resist pattern 7 may completely disappear during this patterning. However, the etching mask 106 consists of a material having a high etching selection ratio with respect to the X-ray absorber 104, and hence serves as a mask if the resist pattern 7 disappears during patterning of the X-ray absorber 104. Thus, the etching mask 106 is useful.
X-ray lithography, which is employed for transferring fine patterns due to the short wavelengths of X-rays, is generally applied to transfer of equal-scale magnification due to the properties of the X-rays. Therefore, high pattern positional accuracy is required of the X-ray mask. If stress remains in the X-ray absorber 104, however, the pattern position thereof is moved by this stress along arrow as shown in FIG. 12 after the patterning, to disadvantageously reduce the positional accuracy.
In the conventional method of fabricating an X-ray mask, therefore, annealing is performed for zeroing the mean membrane stress of the X-ray absorber 104.
However, the etching mask 106 formed in the conventional method of fabricating an X-ray mask generally has a columnar crystal structure. Therefore, the etching mask 106 is readily patterned along grain boundaries through the resist pattern 7 serving as a mask as shown in FIGS. 13A and 13B, to result in edge roughness of the pattern. When the etching mask 106 causing such edge roughness is employed as a mask for patterning the X-ray absorber 104, the pattern is misregistered by a dimension W.sub.0 in FIGS. 13A and 13B, disadvantageously leading to deterioration of the pattern accuracy.
FIG. 13B shows the X-ray mask as viewed from the direction of arrow in FIG. 13A, while omitting the resist pattern 7.
In the conventional method of fabricating an X-ray mask, no consideration is given to stress and stress irregularity of the etching mask 106 and the antireflection film 103a, although the stress of the X-ray absorber 104 is taken into account. Therefore, the positional accuracy is disadvantageously deteriorated after the patterning of the X-ray absorber 104. This problem is now described in detail with reference to the etching mask 106 having stress irregularity.
FIGS. 14 to 17 are schematic sectional views for illustrating the deterioration of the positional accuracy caused by stress irregularity of the etching mask 106 in the conventional method of fabricating an X-ray mask. Referring to FIGS. 14 to 17, the antireflection film 103b and the support ring 5 shown in FIG. 8 are omitted, for convenience of illustration.
Referring to FIG. 14, the mean membrane stress of the X-ray absorber 104 is zeroed by annealing, as hereinabove described.
Referring to FIG. 15, the etching mask 106 having stress irregularity shown in FIG. 18 is formed on the X-ray absorber 104. In this case, a prescribed point A of the X-ray absorber 104 is pulled and moved by the stress of the etching mask 106.
Referring to FIG. 16, the etching mask 106 is patterned through the resist pattern 7 serving as a mask in this state, and the X-ray absorber 104 is patterned through the patterned etching mask 106.
Referring to FIG. 17, the resist pattern 7 and the etching mask 106 are thereafter removed, whereby the point A of the X-ray absorber 104 is released from the stress of the etching mask 106 and restored to the original position. Thus, the pattern position is misregistered by the dimension W.sub.1 of this restoration.